FIELD OF THE INVENTION
The invention relates to a method for temperature compensation of measured values of a turbidity sensor which is built in at some location that is reachable by washing fluid in a water-carrying household appliance, preferably an automatic washing machine or dishwasher, having process control using a microprocessor and containing a light transmitter and a light receiver for transmitting light through the washing fluid located within a measurement path.
The turbidity of the washing fluid can be detected as a process parameter and used to optimize the rinse cycle through the use of a turbidity sensor in the washing machine. Since the turbidity sensor is located in the vicinity of the washing fluid, there is a thermal coupling between the turbidity sensor and the washing fluid. The temperature of the washing fluid may fluctuate in normal operation between about 10.degree. and 95.degree., so that the turbidity sensor is likewise exposed to severe temperature fluctuations.
The turbidity sensor functions according to an optical method and includes a transmitter (such as an LED), which transmits light in the near infrared range, and an optical receiver (phototransistor or photodiode or photoresistor), which converts the optical infrared signal into a proportional electrical signal. However, the transmitter (infrared LED, for instance) and the receiver being used (phototransistor, for instance) are highly temperature-dependent in their electrooptical properties. Without suitable temperature compensation, temperature fluctuations would be interpreted as fluctuations in the turbidity value and would lead to incorrect results in the evaluation of the signal. A temperature compensation of the turbidity sensor is therefore necessary in all appliances in which the turbidity sensor is exposed to major temperature fluctuations, such as in washing machines or dishwashers.
A washing machine with a device for measuring the dirtiness of the washing fluid is known from Published European Patent Application 0 393 311 A1. That device likewise uses an electrooptical turbidity sensor, which is to be calibrated at the beginning of a process that includes a plurality of turbidity measurements. That calibration relates to a reference value for the turbidity which is intended to be equivalent to a measured value definition of zero. To that end, before the measurements begin, the turbidity sensor is exposed to pure tap water or air, and the measurement value thus obtained is defined as zero. However, the known device does not take into account the fact that the ambient temperature of the turbidity sensor can fluctuate sharply and has a considerable influence on the magnitude of the measured turbidity value.
If a combination of a phototransistor and an IR LED is used for the turbidity measurement, then the temperature coefficients of the two components are superimposed on one another, and a total coefficient is obtained for the turbidity sensor. That is also true when similar components are used.
The total temperature coefficient is composed of many influencing variables, which are listed herein as examples for the combination of an IR LED and a phototransistor:
The radiation of the transmitter shifts with increasing temperature to higher wavelengths. The coupling factor between the transmitter and the receiver thus varies as well. PA1 The radiation out of the transmitter drops with decreasing temperature. As a result, the measurement current in the phototransistor drops by the factor according to which the light intensity decreases. PA1 Upon heating up of the IR LED, the on-state voltage drops by approximately 2 mV/K, and as a result if the on-state current remains the same the electrical output or performance drops. The light intensity drops even further as a result. PA1 At a high temperature, the IR LED ages faster, and the consequence is a decreasing light intensity. PA1 The spectral sensitivity of the phototransistor varies with the temperature, resulting in a variation in the coupling factor between the phototransistor and the IR LED. PA1 The current gain factor of the phototransistor increases with a rising temperature. As a result, the measurement current in the phototransistor rises while the light intensity is constant. PA1 The dark current of the phototransistor rises with increasing temperature, resulting in a general raising of the measurement current, regardless of the light intensity. PA1 The phototransistor ages faster at a high temperature, leading to a change in the electrooptical parameters. PA1 The index of refraction of the water in the measurement path varies with the temperature. As a result, the photooptical parameters of the beam path between the transmitter and the receiver vary, which in turn affects the coupling factor.
It is apparent from the many parameters that are dependent on the temperature and have an influence on the measurement current, that one cannot compensate for every influencing variable individually. The turbidity sensor must therefore be considered as an overall system, having parameters which must be determined by trial and error. That is particularly true for the temperature coefficients of the electronic components of the turbidity sensor, because they are very strongly expressed in the measurement result.
In experiments it has been found to be particularly problematic that the ascertained temperature coefficient is also dependent on the particular operating point. That is true both for the receiver (phototransistor, for instance) and the transmitter (IR LED, for instance). In the phototransistor, the temperature coefficient is dependent on the collector-to-emitter voltage, and in the IR LED it is dependent on the on-state current. If the on-state current of the IR LED is constant, then while the temperature coefficient of the IR LED does not vary, nevertheless the collector-to-emitter voltage of the phototransistor and thus its temperature coefficient vary as a function of the turbidity. In the temperature compensation, one can therefore not expect a constant compensation factor but instead must adapt the compensation factor dynamically.
The temperature coefficient of a turbidity sensor, over the temperature range applicable to washing fluids, when the so-called transmitted light method is used, is discussed below with reference to FIG. 1.
The same measurement without making the measurement path turbid, is discussed below with regard to FIG. 2.
Without suitable compensation, such temperature-dictated signal drifts would make the turbidity measurement so markedly wrong that a conclusive measurement of turbidity would no longer be possible.